Per the Regulation (EC) No 1272/2008 of the European Parliament and of the Council on classification, labeling and packaging of substances and mixtures” boric acid (a component of most brazing fluxes), is classified as a reproductive toxin in the European Union. This requires special labeling, leading to an effort on the part of consumers to look for boric acid free alternatives. Suitable boric acid free brazing fluxes must be developed to maintain market share and meet consumer demand.
Brazing is a thermal process, similar to soldering for joining metallic stock materials (hereinafter referred to as base metal), wherein a liquid phase is obtained by melting only the brazing metallic material (hereinafter referred to as filler metal) at temperatures exceeding 450° C. (840° F.). The solidus temperature of the base metal is not reached during this process. Filler metals can be alloys or pure metals.
Assuming pure metal surfaces, the liquid filler metal is able to spread in a thin layer on the base metal surface, wetting it. The filler metal adheres to the base metal surface by a slight alloying of the base and filler metals. The filler metal spreads out over the joint surface and, after solidifying, forms a loadable joint with the base metal.
If designed in a manner suitable for brazing, the two joint surfaces of the parts to be joined form a narrow parallel slit, or capillary. The molten filler metal then flows of its own accord into this slit due to the capillary action, filling said slit. The minimum temperature on the surface of the component to be brazed at which said process takes place undisturbed is the so-called working temperature. It is a characteristic quantity for the filler metal in question.
In order to be able to form a bond with the base metal, the molten filler metal must come into direct contact with the base metal. Oxide layers of the kind present on any engineering metal surface must thus be loosened first and removed. If brazing takes place in the air, this is achieved by covering the brazing site with fluxes in the melt flow of which the oxides dissolve, are reduced or decompose at and above the active temperature of the flux.
The flux thus primarily has the task of removing oxides present on the filler and base metal surfaces and preventing them from re-forming during the brazing process so that the filler metal is able to wet the base metal sufficiently.
The melting point and the effective temperature of the fluxes must be matched to the working temperature of the brazing filler metal used, whereby the flux should melt at about 50-100° C. below the working temperature of the filler metal used and become fully effective from this temperature onwards. Moreover, the molten flux should form a dense, uniform coating on the workpiece which remains intact at the required brazing temperature and for the duration of the brazing period.
Brazing fluxes are composed substantially of salt mixtures which, in the molten state, are capable of dissolving metal oxides. These fluxes are substantially inorganic boron compounds such as, in particular, alkali borates and fluoroborates, including boric acid, and halides such as, in particular, an alkali halide; e.g. alkali fluorides.
At least one aspect of the invention resides in the superior ability to achieve desirable flux characteristics without the presence of boric acid (H3BO3) or borax (NaB4O5(OH)4.H2O) in the flux.